Part 1
Refrigeration and meat quality
1
Microbiology of refrigerated meat
There are many pertinent texts on the microbiology of meats. The purpose
of this chapter is to examine briefly the types of micro-organisms and con-
ditions that are of interest in relation to the refrigeration of meat and meat
products.
In a perfect world, meat would be completely free of pathogenic (food
poisoning) micro-organisms when produced. However, under normal
methods the production of pathogen-free meat cannot be guaranteed. The
internal musculature of a healthy animal is essentially sterile after slaughter
(Gill, 1979, 1980). However, all meat animals carry large numbers of differ-
ent micro-organisms on the outer surfaces of the body and in the alimentary
tract. Only a few types of bacteria directly affect the safety and quality of
the finished carcass. Of particular concern are foodborne pathogens such
as Campylobacter spp., Clostridium perfringens, pathogenic serotypes of
Escherichia coli, Salmonella spp., and Yersinia enterocolitica.
In general, the presence of small numbers of pathogens is not a problem
because meat is normally cooked before consumption. Adequate cooking
will substantially reduce the numbers, if not completely eliminate all of the
pathogenic organisms present on the meat. Most meat-based food poison-
ing is associated with inadequate cooking or subsequent contamination after
cooking.The purpose of refrigeration is to reduce or eliminate the growth of
pathogens so that they do not reach levels that could cause problems.
Normally the growths of spoilage organisms limit the shelf-life of meat.
The spoilage bacteria of meats stored in air under chill conditions include
species of Pseudomonas, Brochothrix and Acinetobacter/Moraxella.In
general, there is little difference in the microbial spoilage of beef, lamb, pork
and other meat derived from mammals (Varnam and Sutherland, 1995).
Meat is considered spoiled by bacteria when the products of their meta-
bolic activities make the food offensive to the senses of the consumer (Gill,
1983). Therefore, the perception of a state of spoilage is essentially a sub-
jective evaluation that will vary with consumer expectations. Few, however,
would not acknowledge that the appearance of slime, gross discoloration
and strong odours constitute spoilage.
‘Off’ odours are due to an accumulation of malodorous metabolic prod-
ucts, such as esters and thiols. Several estimations have been made of the
number of bacteria on meat at the point at which odour or slime becomes
evident and the mean is about 3 ￥ 10
7
cm
-2
(Shaw, 1972).When active growth
occurs, the number of bacteria increases exponentially with time.Therefore,
a convenient measure of the growth rate is the time required for doubling
of numbers, often called the generation time. If this, for example, were one
hour, the number would increase two-fold in 1 h, four-fold in 2 h, eight-fold
in 3 h, and so on.
The bacterial safety and rate of spoilage depends upon the numbers and
types of micro-organisms initially present, the rate of growth of those micro-
organisms, the conditions of storage (temperature and gaseous atmosphere)
and characteristics (pH, water activity a
w
) of the meat. Of these factors, tem-
perature is by far the most important.
1.1 Factors affecting the refrigerated shelf-life of meat
1.1.1 Initial microbial levels
1.1.1.1 Tissue sterility
For many years microbiologists believed that the tissues of healthy animals
normally contained bacteria (Reith, 1926; Ingram, 1972). These ‘intrinsic’
bacteria were the cause of phenomena such as ‘bone taint’. The cause of
bone taint is still questioned and will be discussed later.The prevailing view
of the majority of textbooks (Banwart, 1989;Varnam and Sutherland, 1995),
based in part on the work of Gill (Gill, 1979, 1980) is that the meat of a
healthy animal is essentially sterile. Low numbers of specific micro-
organisms, which have reached the tissues during the life of the animal, may
occur in the viscera and associated lymph nodes from time to time (Gill,
1979; Roberts and Mead, 1986). These are often pathogenic species, such as
Salmonella, and clostridia spores.The absence of bacteria appears to be due
to the continued functioning of the immune system in slaughtered animals.
Experiments with guinea pigs showed that the antibacterial defences of live
animals persisted for an hour or more after death and could inactivate
bacteria introduced during slaughter (Gill and Penney, 1979). Clearly, if
bacteria are thus inactivated there can be no multiplication, in deep tissue,
during carcass chilling irrespective of cooling rates.
4 Meat refrigeration
1.1.1.2 Rigor mortis
The way in which animals are handled before slaughter will effect the bio-
chemical processes that occur before and during rigor mortis. The resulting
metabolites influence the growth of micro-organisms on meat.
During the onset of rigor mortis, which may take up to 24 h, oxygen
stored in the muscle is depleted and the redox potential falls from above
+250 mV to -150 mV. Such a low redox value combined with the initial
muscle temperature of 38 °C provides ideal growth conditions for meso-
philic micro-organisms. Stress and excitement caused to the animal before
slaughter will cause the redox potential to fall rapidly, possibly allowing
proliferation of such micro-organisms before cooling (Dainty, 1971).
Concurrent with the fall in redox potential is a fall in pH from an initial
value in life of around 7 to a stable value around 5.5, the ‘ultimate pH’. This
is due to the breakdown of glycogen, a polysaccharide, to lactic acid in the
muscle tissue. Lactic acid cannot be removed by the circulation nor oxi-
dised, so it accumulates and the pH falls until the glycogen is all used or
the breakdown stops. The pH has an important role in the growth of micro-
organisms, the nearer the pH is to the ultimate value, the more growth is
inhibited (Dainty, 1971). Stress or exercise before slaughter can deplete an
animal’s glycogen reserves, consequently producing meat with less lactic
acid and a relatively high ultimate pH, this gives the meat a dark, firm, dry
(DFD) appearance. Alternative terms are ‘dark cutting’ and ‘high-pH
meat’. The condition occurs in pork, beef and mutton, but is of little eco-
nomic importance in the latter (Newton and Gill, 1981). DFD meat pro-
vides conditions that are more favourable for microbial growth than in
normal meat. The microbiology of DFD meat has been comprehensively
reviewed by Newton and Gill (1981).
Glucose is the preferred substrate for growth of pseudomonads, the
dominant bacteria in meat stored in air at refrigerated temperatures. Only
when glucose is exhausted do they break down amino acids, producing the
ammonia and sulphur compounds that are detectable as spoilage odours
and flavours. In meat containing no glucose, as is the case with some DFD
meat, amino acids are broken down immediately and spoilage becomes
evident at cell densities of 6 log
10
cfucm
-2
(colony forming units per cen-
timetre squared). This is lower than in normal meat, where spoilage
becomes apparent when numbers reach ca. 8log
10
cfucm
-2
. Thus, given the
same storage conditions, DFD meat spoils more rapidly than normal-pH
meat. There is no evidence that the spoilage of pale, soft, exuding (PSE)
meat is any different to that of normal meat (Gill, 1982). There is little sig-
nificant difference in pH or chemical composition between PSE and normal
meat.
1.1.1.3 Surface contamination
Initial numbers of spoilage bacteria on carcasses significantly affect shelf-
life. With higher numbers, fewer doublings are required to reach a spoilage
Microbiology of refrigerated meat 5
level of ca.10
8
organisms cm
-2
. For example, starting with one organism
cm
-2
, 27 doublings would be needed; for 10
3
organisms cm
-2
initially, the
number of doublings is reduced to 17.
Contamination of carcasses may occur at virtually every stage of slaugh-
tering and processing, particularly during flaying and evisceration of red-
meat animals and scalding, and mainly affects the surface of the carcass.
Sources of contamination have been reviewed by James et al. (1999).
Hygienic handling practices should ensure that total viable counts on the
finished carcass are consistently 10
3
–10
4
organisms cm
-2
or lower for red
meats. Bad practices can cause counts to exceed 10
6
organisms cm
-2
.
With red meats, carcasses of good microbial quality are obtained by
1 preventing contamination from the hide;
2 avoiding gut breakage;
3 the adoption of good production practices that include more humane
practices throughout the slaughtering system.
The effectiveness of chemical and physical decontamination systems for
meat carcasses has been reviewed by James and James, (1997) and James
et al. (1997). Commercial systems using steam have been introduced into
the USA and are claimed to reduce the number of bacteria on the surface
of beef carcasses to below 1 log
10
cfucm
-2
(Phebus et al., 1997).
1.1.2 Temperature
Micro-organisms are broadly classified into three arbitrary groups (psy-
chrophiles, mesophiles and thermophiles) according to the range of tem-
peratures within which they may grow. Each group is characterised by three
values: the minimum, optimum and maximum temperatures of growth.
Reduction in temperature below the optimum causes an increase in genera-
tion time, i.e. the time required for a doubling in number. It is an accepted
crude approximation that bacterial growth rates can be expected to double
with every 10 °C rise in temperature (Gill, 1986). Below 10°C, however, this
effect is more pronounced and chilled storage life is halved for each 2–3°C
rise in temperature. Thus the generation time for a pseudomonad (a
common form of spoilage bacteria) might be 1h at 20°C, 2.5 h at 10°C, 5 h
at 5 °C, 8 h at 2 °C or 11 h at 0 °C. In the usual temperature range for chilled
meat, -1.5–+5 °C, there can be as much as an eight-fold increase in growth
rate between the lower and upper temperature. Storage of chilled meat at
-1.5 ± 0.5 °C would attain the maximum storage life without any surface
freezing.
Meat stored above its freezing point, ca. -2 °C, will inevitably be spoiled
by bacteria. Obviously, the nearer the storage temperature of meat
approaches the optimum for bacterial growth (20–40 °C for most bacteria)
the more rapidly the meat will spoil.Work of Ayres (1960) compared the rate
of increase in bacterial number on sliced beef stored at 0, 5, 10, 15, 20 and
6 Meat refrigeration
25 °C. The meat developed an off odour by the third day at 20 °C, the tenth
day at 5 °C and the 20th day at 0 °C. Similar data has been reported by other
workers.They clearly demonstrate the effectiveness of refrigeration in reduc-
ing the rate of increase in bacterial numbers and extending shelf-life.
As bacteria generally grow more rapidly than fungi, mould spoilage of
meat is thought to develop only when competing bacteria are inhibited.
Temperature is usually assumed to be the critical factor, mould spoilage
being typically associated with frozen meat. It has been generally accepted
that moulds can develop on meat at temperatures as low as -10 or -12 °C.
There is some evidence that this is an exaggeration and that for practical
purposes the minimum temperature for mould growth on meat should be
taken to be ca. -5 °C (Lowry and Gill, 1984). It is further thought that
surface desiccation, rather that temperature, is the factor that inhibits bac-
terial growth. If this is the case then mould growth on frozen meats is indica-
tive of particularly poor temperature control.
Many factors influence the growth and survival of micro-organisms in
meat during freezing and frozen storage. However, the main factor affect-
ing the growth of micro-organisms during freezing is the availability of
water. Until the temperature is reduced below the minimum temperature
for growth, some micro-organisms have the potential to multiply. While
most of the water in meat is turned to ice during freezing, there is always
some free liquid water available, 26% at -5°C, 18% at -10°C, 14% at
-18°C, 10% at -40 °C (Rosset, 1982). The transformation of water into ice
significantly modifies the growth environment for micro-organisms, since
solutes become concentrated in the remaining free water to the level that
microbial growth is inhibited. Below the freezing point of the meat, the
water activity is progressively reduced preventing microbial growth (Fig.
Microbiology of refrigerated meat 7
–30 –20 –10 0
Temperature (°C)
0.7
0.8
0.9
1.0
W
ater activity (
a
w
)
Fig. 1.1 Water activities (a
w
) of meat at various sub-freezing temperatures (source:
Leistner and R?del, 1976).
1.1). The greatest reduction in the microbial load occurs during, or shortly
after, freezing itself. During frozen storage, the numbers are gradually
reduced further.
1.1.2.1 Pathogenic organisms
A number of bacterial pathogens capable of causing food poisoning in
humans are known to contaminate red meat. Those of most importance
are Campylobacter spp., Clostridium perfringens, pathogenic serotypes of
Escherichia coli (principally E. coli O157:H7), Salmonella spp. and Yersinia
enterocolitica (Nottingham, 1982; Anon, 1993; Mead and Hinton, 1996). Lis-
teria monocytogenes is commonly associated with meat, but its public health
significance in relation to raw meat is unclear (Mead and Hinton, 1996).
The essential characteristics of pathogenic micro-organisms can be found
in numerous texts.
Minimum and optimum growth temperatures for pathogens commonly
associated with red meat are show in Table 1.1. Some pathogens, such as
L. monocytogenes, are capable of growth at chill temperatures below 5 °C.
These are often cited as being of particular concern in relation to refriger-
ated meats since refrigeration can not be relied on to prevent growth
(Doyle, 1987). On the other hand, psychrotrophic pathogens are not par-
ticularly heat resistant and adequate cooking should be sufficient to destroy
any such pathogens. Illnesses caused by L. monocytogenes and E. coli are
often due to inadequate cooking before ingestion.
1.1.2.2 Spoilage organisms
The number of types of micro-organisms capable of causing food spoilage
is very large and it is not possible to discuss them in any detail in this text.
Depending on the initial microflora and the growth environment, only a
few species of the genera Pseudomonas, Acinetobacter, Moraxella,
Lactobacillus, Brochothrix and Alteromonas, and of the family
8 Meat refrigeration
Table 1.1 Minimum and optimum growth temperatures for pathogens associated
with red meats
Minimum temperature Optimum temperature
(°C) (°C)
Campylobacter spp. 30 42–43
Clostridia perfringens 12 43–47
Pathogenic Escherichia 7 35–40
coli strains
Salmonella spp. 5 35–43
Listeria monocytogenes 0 30–37
Yersinia enterocolitica -2 28–29
Source: Mead and Hinton, 1996.
Enterobacteriaceae are significantly represented in most spoilage
microflora of chilled meats (Bell and Gill, 1986).
The micro-organisms that usually spoil meat are psychrotrophs, i.e. they
are capable of growth close to 0 °C. Only a small proportion of the initial
microflora on meat will be psychrotrophs; the majority of micro-organisms
present are incapable of growth at low temperatures. As storage tempera-
ture rises the number of species capable of growth will increase.
1.1.2.2.1 Spoilage of chilled meat
The spoilage of chilled meat stored in air is dominated by Gram-negative,
psychrotrophic, aerobic rod-shaped bacteria. Although a wide range of
genera are present on meat, only Pseudomonas, Acinetobacter and
Psychrobacter species are normally of importance (Dainty and Mackey,
1992). Of these, species of Pseudomonas are of greatest importance (Gill,
1986). Pseudomonas spp. typically account for >50% of the flora and some-
times up to 90% (Dainty and Mackey, 1992).
Other bacteria are present in small numbers and may occasionally form
a significant part of the microflora. Brochothrix thermosphacta appears to
be of more importance on pork and lamb than on beef especially on fat
where the pH value is generally higher, and at temperatures above 5 °C
(Gill, 1983; Varnam and Sutherland, 1995).
Species of both Micrococcus and Staphylococcus are present on meat
stored in air but their significance is generally considered limited under
refrigerated storage. Psychrotrophic members of the Enterobacteriaceae,
including Serratia liquefaciens, Enterobacter agglomerans and Hafnia alvei
are also common at low levels (Dainty and Mackey, 1992). These organisms
become of greater importance at temperatures of 6–10°C, but Pseudo-
monas spp. usually remain dominant (Varnam and Sutherland, 1995).
Yeasts and moulds are considered by many to be of limited importance
in modern practice (Varnam and Sutherland, 1995). Moulds were of historic
importance on carcass meat stored for extended periods at temperatures
just above freezing.
1.1.2.2.2 Spoilage of chilled packaged meat
Large vacuum packs usually contain ca. 1% O
2
that in theory will support
the growth of pseudomonads (Varnam and Sutherland, 1995). Continuing
respiration, however, by the meat rapidly depletes oxygen (O
2
) and
increases the carbon dioxide (CO
2
) concentration to ca. 20%. Pseudomonas
spp. are usually unable to grow under such conditions. In general conditions
vacuum packs favour lactic acid bacteria (LAB), although there may also
be significant growth of Br. thermosphacta,‘Shewanella putrefaciens’ (for-
mally Altermonas putrefaciens) and the Enterobacteriaceae. Under anaer-
obic conditions LAB have a considerable advantage in growth rate over
competing species of facultative anaerobes (Fig. 1.2). The predominant
LAB are homofermentative species of Lactobacillus, Carnobacterium and
Microbiology of refrigerated meat 9
Leuconostoc. Lactococcus spp. are much less common. LAB are able to
grow at low temperatures and low O
2
tensions and tolerate CO
2
. Psy-
chrophilic species of Clostridium have been recognised as a significant
potential problem (Varnam and Sutherland, 1995).
Both Br. thermosphacta and Sh. putrefaciens are favoured by high pH
values. Sh. putrefaciens is unable to grow below pH 6.0 during storage at
low temperatures, whereas Br. thermosphacta is unable to grow anaerobi-
cally below pH 5.8 (Gill, 1983). At temperatures below 5 °C, Enterobacte-
riaceae are inhibited in vacuum packs by CO
2
, low pH and lactic acid. At
higher temperatures and pH values, CO
2
is markedly less inhibitory and
growth is possible, in particular by Serratia liquefaciens and Providencia spp.
(Varnam and Sutherland, 1995).
The predominant type of spoilage in vacuum-packed chilled meat is
souring (Sofos, 1994; Varnam and Sutherland, 1995). This is not normally
detectable until bacterial numbers reach 8 log
10
cfucm
-2
or greater. The
exact cause of such spoilage is unknown, but is assumed to result from lactic
acid and other end-products of fermentation by dominant LAB. High-pH
(DFD) vacuum-packed meat spoils rapidly and involves the production of
large quantities of hydrogen sulphide (H
2
S) by Sh. putrefaciens and Enter-
obacteriaceae (Gill, 1982; Varnam and Sutherland, 1995). Characteristic
‘greening’ occurs owing to H
2
S combining with the muscle pigment to
give green sulphmyoglobin; the meat also develops putrid spoilage odours
(Gill, 1982).
Packaging in various gaseous atmospheres has been used as an alterna-
tive to vacuum packing. The intention has been to preserve the fresh meat
colour and to prevent anaerobic spoilage by using high concentrations of
10 Meat refrigeration
15 10 5 2
0
10
20
30
40
50
60
Gener
ation time (h)
Temperature (°C)
Br. thermosphacta
Enterobacter
Lactobacillus
Fig. 1.2 Effect of temperature on the rates of anaerobic growth of bacteria on meat
slices (source: Newton and Gill, 1978).
oxygen (50–100%) along with 15–50% carbon dioxide to restrict the growth
of Pseudomonas and related species (Nottingham, 1982). The microflora of
meat stored in commercially used modified atmosphere packs (MAP) is in
general similar to that of vacuum packs (Varnam and Sutherland, 1995). At
temperatures below 2 °C, LAB are dominant, Leuconostoc spp. being the
most important. Br. thermosphacta, Pseudomonas spp. and Enterobacteri-
aceae are more prevalent in MAP (modified atmosphere packs) than
vacuum packs at storage temperatures ca. 5 °C, rather than 1 °C. Br. ther-
mosphacta is relatively CO
2
tolerant and the presence of O
2
permits growth
of this bacterium at pH values below 5.8. Prior conditioning in air favours
the growth of these bacteria, they are also more prevalent in pork than
other meats (Dainty and Mackey, 1992). The spoilage of meat in MAP may
involve souring similar to that in vacuum-packed meat. Other characteris-
tics include ‘rancid’ and ‘cheesy’ odours. Chemical rancidity does not appear
to be primarily involved and souring is probably caused by the metabolites
of LAB or Br. thermosphacta (Varnam and Sutherland, 1995).
1.1.2.2.3 Spoilage of frozen meat
Micro-organisms do not grow below ca. -10 °C, thus spoilage is only nor-
mally relevant to handling before freezing or during thawing. In these
contexts, frozen meats behave like their unfrozen counterparts, although
growth rates may be faster after thawing, owing to drip.
Although Salmonella, Staphylococci, and other potential pathogens can
survive freezing and frozen storage, the saprophytic flora (spoilage bacte-
ria) tend to inhibit their growth (Varnam and Sutherland, 1995). During
freezing and thawing of food, the temperature favours the growth of
psychrophilic organisms, most of which are spoilage organisms. Hence,
in nearly all cases, if a frozen product is mishandled, spoilage is apparent
before the food becomes a health hazard.
In the past, carcass meats were imported at temperatures of -5 to
-10°C. At these temperatures there were problems with the growth of psy-
chrotrophic moulds such as strains of Cladosporium, Geotrichum, Mucor,
Penicillium, Rhizopus and Thamnidium, causing ‘whiskers’ or ‘spots’ of
various colours depending on the species. Since little meat is now stored
at these temperatures mould spoilage is largely of historic importance.
Despite this, many meat microbiology textbooks continue to discuss this
subject in great detail.
1.1.3 Relative humidity
Historically low relative humidities (RH) have been recommended to
extend shelf-life. Schmid (1931) recommended a storage temperature for
meat of 0 °C and an RH of 90%. Haines and Smith (1933) later demon-
strated that lowering the RH is more effective in controlling bacterial
growth on fatty or connective tissue than on lean meat. This was due to a
slower rate of diffusion of water to the surface. Low RH was more
Microbiology of refrigerated meat 11
effective, therefore, in reducing microbial spoilage of carcass meat than of
small lean joints.
Micro-organisms normally grow in foods in the equivalent of a nutrient
solution and the availability of water in this solution is one of the factors
normally controlling growth. The term used for physiologically available
water is ‘water activity’ (a
w
). By definition
Where P and P
0
are the vapour pressures of the solution and of the pure
solvent, respectively. The above equation also defines the relative humidity
(RH) of the vapour phase in equilibrium with the solution. The RH (%) of
an atmosphere in equilibrium with a food would be 100 times the a
w
of the
food.
If the RH of the atmosphere corresponds to an a
w
lower than that of
meat then the meat surface will lose water to the atmosphere and the a
w
will fall. In practice, the a
w
of the surface of meat will not fall to the corre-
sponding RH value of the atmosphere, because water lost from the surface
is partially replaced by diffusion of water from the interior. However, the
basis of the efficacy of low relative humidities in extending shelf-life is the
reduction of the a
w
of the meat surface to a level inhibiting the growth of
psychrophilic bacteria.
The a
w
of lean meat is of the order of 0.993 (Scott, 1936) and offers ideal
growth conditions for micro-organisms. Scott (1936) and Scott and Vickery
(1939) established that the important meat spoilage bacteria are unable to
grow on meat at temperatures below 4°C if the a
w
is less than 0.96. This
occurs when the water content has fallen from about 300 to about 85 g
of water/100 g of dry matter. They recommended that in chilled storage
the maximum values of the mean RH at air speeds of 0.15, 0.45, 0.70 and
0.90ms
-1
should be approximately 72, 85, 88 and 90%, respectively. These
conditions would maintain the water content at the surface of the meat
at or near inhibitory levels for bacteria. If conditions during the cooling
stage were unsatisfactory, they recommended that for any given air speed
the relative humidity would need to be lower to prevent bacterial growth.
These recommendations were made for long-term storage during ship-
ment from Australia to the UK. Higher relative humidities may be used if
only short-term storage is the aim.This work remains the basis for the usual
recommendation to operate meat chillers between 85–95%. The actual RH
used will depend, of course, upon the air speed, the type of meat, the length
of storage required and the temperature of storage.
1.2 Other considerations
Legislation and recommendations for cooling of meat are believed to be
based on clear microbiological criteria. However, there are a lack of data
aPP
w
=
0
12 Meat refrigeration
to support recommendations on the avoidance of bone taint, and on chill-
ing rates for hot and cold boning.
1.2.1 Bone taint
It has been said (Moran and Smith, 1929) that ‘possibly the strongest argu-
ment for the rapid cooling of beef immediately after death is that it reduces
the possibility of bone taint’. Twenty years earlier in the USA, Richardson
and Scherubel (1909) had concluded that, to control the condition, the
carcass should be cooled to 4 °C or below at the centre of the round
(hindquarter) within 48 h of slaughter.
Bone taint has long been regarded as evidence for the presence of intrin-
sic bacteria. This view is diminishing, but the exact nature of what consti-
tutes bone taint remains undefined (Nottingham, 1982; Shaw et al., 1986).
A wide range of bacteria has been implicated in the past. Varnam and
Sutherland (1995) in their book on meat report that halophilic Vibrio
species and Providencia are now most commonly attributed to this condi-
tion, while work by De Lacy et al. (1998) demonstrated that some strains
of psychrotrophic Clostridium spp. have the potential to cause bone taint.
Bone taint in beef is usually localised in the region of the hip joint and
is manifested by a ‘typical sewage type odour’ or ‘putrefactive sulphide-type
odours’. This is referred to as ‘souring’ in the American literature, which is
only detected when the hindquarter is divided (Thornton, 1951). Taints
in pork products appear to differ from bone taint in beef, occurring most
often after processing into cured hams or gammons (Jensen and Hess, 1941;
Haines, 1941).
The aetiology of bone taint remains obscure.The bacteria associated with
bone taint are supposed to have their origin in the bloodstream at death
and the infection starts in the blood vessels of the marrow of the femur
(Callow and Ingram, 1955). How they get into the blood supply is not estab-
lished. One possibility is that unusually large numbers of bacteria are
introduced at slaughter, for example, by the use of slaughter instruments
contaminated with faeces (Jensen and Hess, 1941; Mackey and Derrick,
1979). Such massive contamination might be sufficient to overwhelm the
immune system leading to survival of a few cells (Gill and Penney, 1979).
Alternatively, bacteria may originate from undetected infection, for
example of joints. The lymph nodes have also been implicated as centres of
infection (Lepovetsky et al., 1953; Cosnett et al., 1956; Nottingham, 1960).
Surprisingly, in view of the early recognition of the importance of
temperature, there are apparently no definitive data on the cooling rate
required to assure freedom from bone taint in the various species of meat
animal. In all examples studied in cattle and sheep (i.e. ruminants) it is
agreed that bone taint will only manifest itself if cooling of the carcass post-
mortem is insufficiently rapid (Kitchell, 1972). Some conditions under
which bone taint has occurred are given in Table 1.2. With pigs, on the other
Microbiology of refrigerated meat 13
hand, Jensen and Hess (1941) reported that, even under optimum cooling
conditions, 6–7.5% of several thousand hams exhibited various forms of
‘souring’. Unfortunately, the cooling data furnished in support of this state-
ment include no temperatures measured at the hip joint.
From time to time bone taint of carcasses is still reported within the
industry. The rarity of bone taint may reflect a requirement for coincident
circumstances, each of which is uncommon. Microbial survival and growth
would be favoured by very high levels of contamination, possible weaken-
ing of the antimicrobial defences (as can occur in haemorrhagic shock), high
muscle pH and slow cooling. To allow for the infrequent occurrence of
bacteria in deep tissues it is thus prudent to cool carcasses promptly after
slaughter. The temperature of the deep leg should be brought below 15 °C
within 24 h.
1.2.2 Cold deboning
Council Directives of the EU (Council Directive No. 64/433/ECC, 1964;
Council Directive No. 83/90/ECC, 1983) stipulate that carcass meat must be
chilled immediately after post-mortem inspection. Chilling must continue
to an internal temperature of 7 °C before cutting or transportation can take
place. This requirement, aimed at preventing growth of salmonellas, has
caused problems in the meat industry. To allow deboning 24 h post-mortem,
the outer portions of the carcass may have to be cooled at a rate causing
toughening due to ‘cold shortening’ and surface fat may become very hard
and difficult to handle. The 1964 Council Directive also stipulated a
maximum cutting room temperature of 10 °C, which was increased to 12 °C
in the 1983 amendments to that Directive.
To judge the need for such a stringent regulation the effect of tempera-
ture on the growth of salmonellas on meat has been defined. Mackey et al.
(1980) used observed generation times of salmonellas on beef surfaces
maintained at high RH values to calculate the maximum extent of growth
during storage for different times at temperatures between 10 and 15 °C.
Smith (1985) produced tables of lag and generation times which can be used
to determine the length of time raw chilled meat can be held at tempera-
tures between 10 and 40 °C without an increase in salmonella numbers.
14 Meat refrigeration
Table 1.2 Cooling conditions under which bone-taint in beef has been reported
Reference Deep muscle temperature Time post-mortem
(°C) (h)
Moran and Smith (1929) 22 24
17 48
Haines and Scott (1940) >18 40 or longer
De Lacy et al. (1998) 20 20
Because of their studies, Mackey et al. (1980) concluded that under prac-
tical conditions of cutting and packaging that takes only 2–3 h, a meat
temperature of 10 °C would be entirely adequate to ensure no significant
multiplication of salmonella. Smith (1985) concluded that cutting rooms
could be maintained above 10 °C provided carcasses are processed
promptly and meat is not allowed to accumulate in the cutting room.
1.2.3 Hot deboning
Hot deboning has received much attention in recent years because of its
potential to streamline butchery, packaging and chilling. As the name
implies the carcass is deboned while hot and the meat is chilled in vacuum
packs or cartons. In principle, this could increase the risk of microbial
growth because there is no surface drying, and some contaminated surfaces
are deep in the meat where they will cool more slowly.
The possible microbiological effects of hot deboning have been investi-
gated either by monitoring growth following natural contamination or by
prediction of growth based on data obtained following inoculation. Natural
contamination experiments have shown that it is possible to produce hot
deboned meat of the same microbiological quality as cold deboned meat
in terms of total counts and numbers of mesophilic pathogenic bacteria
(Taylor et al., 1980). Based on natural contamination experiments, Fung
et al. (1981) recommended chilling to 21°C within 3–9h after packaging
with continuous chilling to below 10 °C within 24 h. The validity of using
natural contamination to monitor the growth of pathogens on hot deboned
meat has, however, been questioned (Grau, 1983) because initial numbers
are very low, often undetectable, and heterogeneous in distribution. As an
alternative, prediction of growth at different cooling rates can be calculated
from an equation derived from observations on the growth of the organ-
ism on meat following inoculation (Herbert and Smith, 1980). Cooling rates
recommended on this basis are more rapid (e.g. cool to 8 °C from an initial
30 °C within 6 h of the commencement of boning) than those based on
natural contamination experiments.
Microbiologists have yet to agree whether the observational (natural
contamination) or predictive approach is more appropriate in defining the
effect of different cooling rates on the extent of microbial growth. Predic-
tion using mathematical models based on data from inoculation experi-
ments avoids the need to perform time-consuming tests on each cooling
rate in question. In some instances, where factors affecting growth are fully
understood, it may even be possible to predict growth with reasonable pre-
cision from growth rate data obtained in laboratory media, as demonstrated
by Gill (1984) for E. coli in tub-packed livers. The accuracy of all predictive
models must, however, be confirmed as far as is possible by comparison with
observed growth at a selection of cooling rates following natural contami-
nation, as performed by Gill (1984).
Microbiology of refrigerated meat 15
1.3 Conclusions
1 The muscle tissue of live, healthy animals is sterile. The source of micro-
bial contamination is usually the hide and to a lesser extent the gut
and occurs during slaughter and handling. Post-process handling is the
usual source of microbial contamination on cooked products, provided
adequate processing has taken place.
2 The rate of spoilage depends upon the numbers and types of organisms
initially present, the conditions of storage (temperature and gaseous
atmosphere) and characteristics (pH, a
w
) of the meat.
3 Spoilage is characterised by off-odours, slime formation and dis-
colouration. The pattern of spoilage is defined by the type of micro-
organisms present. The dominance and thus type of spoilage is
dependent on the storage conditions.
4 Those bacteria responsible for the spoilage of carcass meat grow most
rapidly above 20 °C. Any reduction below this temperature will extend
the storage life. Broadly speaking bacterial growth will be half as fast at
5 °C as at 10 °C and half as fast again at 0 °C, i.e. meat should keep
roughly four times longer at 0 °C than at 10 °C.
5 The precise reasons for bone taint are still not fully understood.
However, the carcass should be chilled as rapidly as possible to an inter-
nal temperature below 15 °C and, finally, to below 5°C within 48 h if the
condition is to be avoided.
6 In evaluating the microbiological consequences of hot deboning there
is a disconcerting difference between recommended ‘safe’ cooling rates
proposed from experiments using naturally contaminated meat and
those using artificially inoculated meat.
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